The invention relates an air filter system of a motor vehicle, comprising an air filter housing with a housing part of moisture-sensitive plastic material and an air passage section with a circumferentially extending passage wall, wherein the passage wall of the air passage section is integrally formed of identical material on the housing part of the air filter housing and wherein a mass air flow sensor is arranged in the air passage section.
Internal combustion engines of a motor vehicle comprise an electronic control unit in particular for metering the fuel quantity to be injected. This control unit operates with the aid of various sensors for determining the operating parameters. An important sensor in this connection is the mass air flow sensor that is embodied in particular as a hot-film mass air flow sensor (HF MAF). The mass air flow sensor is arranged in the air intake passage and supplies the engine control unit with data in regard to the actually drawn in mass air flow. A preferred attachment location for the mass air flow sensor is the filtered air side of the air filter housing where the sensitive mass air flow sensor is exposed to a filtered intake air flow. Known HF MAF sensors however are not only sensitive with regard to soiling but, for providing precise measuring results, require also a laminar and turbulence-free flow that moreover must be precisely defined with regard to its geometric course.
For ensuring a good measuring result, configurations of the HF MAF sensor are known that provide the sensor with its own housing that is inserted into the air filter housing. Such configurations however incur comparatively high costs. Moreover, the interface of HF MAF sensor housing and air filter housing causes a reduction of the free flow cross-section so that an undesirably increased pressure loss in the intake air flow is observed.
For avoiding the above disadvantages an alternative configuration is known where in particular a housing part of the air filter housing at the filtered air side is provided with an integrally formed air passage section of material identical to that of the housing part wherein in this air passage section the mass air flow sensor is arranged and extends through the passage wall of the air passage section. Such mass air flow sensors that are also referred to as plug-in HF MAF sensors do not have their own housing. There is no cross-sectional loss as a result of the interface to the air filter housing so that the pressure loss in the intake air flow is reduced. Also, the costs of the arrangement are reduced.
However, in this connection several difficulties have to be overcome. The air filter housing including the integrally formed air passage section of identical material is typically produced of a plastic material such as polyamide or the like. In this connection, in addition to the unavoidable thermal dimensional changes also moisture-caused dimensional changes occur because polyamide, as a moisture-sensitive plastic material, has the tendency to absorb moisture. The moisture-caused dimensional changes of the air passage section supporting the mass air flow sensor cannot be compensated easily. As a result, moisture-caused dimensional changes of the air passage section affect the geometric flow course and thus cause measuring errors or measuring imprecisions. A possibly required flow guiding grid that is provided for calming the flow or for causing laminar flow of the intake air flow and that contributes therefore to measuring precision must be manufactured and mounted as a separate component; this is not optimal with regard to costs and also generates additional dimensional tolerances.
It is therefore an object of the present invention to improve an air filter system of the aforementioned kind in such a way that an increased measuring precision of the mass air flow sensor is provided.